Figure 1a. Maximum efficiency
of a "drag" device is obtained
when the collector is pushed
away from the wind, as is a
simple, drag-type sail boat. In
this Persian panemone design,
the rotor can only harvest half
of the wind striking the collection
area. The panemone is one of
the least efficient, but most
commonly reinvented (and
patented) wind turbine concepts.

The history of wind power
shows a general evolution from the use of simple, light devices driven by aerodynamic drag
forces; to heavy, material-intensive drag devices; to the increased use of light,
material-efficient aerodynamic lift devices in the modern era. But it shouldn't be
imagined that aerodynamic lift (the force that makes airplanes fly) is a modern concept
that was unknown to the ancients. The earliest known use of wind power, of course, is the
sail boat, and this technology had an important impact on the later development of
sail-type windmills. Ancient sailors understood lift and used it every day, even though
they didn't have the physics to explain how or why it worked.

The first windmills were
developed to automate the tasks of grain-grinding and water-pumping and the earliest-known
design is the vertical axis system developed in Persia about 500-900 A.D. The first use
was apparently water pumping, but the exact method of water transport is not known because
no drawings or designs -- only verbal accounts -- are available. The first known documented
design is also of a Persian windmill, this one with vertical sails made of bundles of
reeds or wood which were attached to the central vertical shaft by horizontal struts
(see
Figure 1a). A 19th Century American approximation of this panemone device is shown
at the left (Figure 1b).

Grain grinding was the first documented
wind mill application and was very straightforward. The grinding stone was affixed to the
same vertical shaft. The mill machinery was commonly enclosed in a building, which also
featured a wall or shield to block the incoming wind from slowing the side of the
drag-type rotor that advanced toward the wind.

Vertical-axis windmills
were also used in China, which is often claimed as their birthplace. While the belief that
the windmill was invented in China more than 2000 years ago is widespread and may be
accurate, the earliest actual documentation of a Chinese windmill was in 1219 A.D. by the
Chinese statesman Yehlu Chhu-Tshai. Here also, the primary applications were apparently
grain grinding and water pumping.

Figure
1b. A 19th-century American knock-off of the Persian panemone that probably
made a
wonderful clothes dryer.

Figure 2. Water Pumping Sailwing Machines on
the Island of Crete

One of the most
scenic and successful applications of windpower (and one that still exists), is the
extensive use of water pumping machines on the island of Crete. Here, literally hundreds
of sail-rotor windmills pump water for crops and livestock.

Figure 3. An early sail-wing horizontal-axis
mill on the Mediterranean coast.

The first windmills to
appear in western Europe were of the horizontal-axis configuration. The reason for the
sudden evolution from the vertical-axis Persian design approach is unknown, but the fact
that European water wheels also had a horizontal-axis configuration -- and apparently served
as the technological model for the early windmills -- may provide part of the answer.
Another reason may have been the higher structural efficiency of drag-type horizontal
machines over drag-type vertical machines, which (remember) lose up to half of
their rotor collection area due to shielding requirements. The first illustrations (1270 A.D.) show a four- bladed mill
mounted on a central post (thus, a "postmill") which was
already fairly technologically advanced relative to the Persian mills. These mills used
wooden cog-and-ring gears to translate the motion of the horizontal shaft to vertical
movement to turn a grindstone. This gear was apparently adapted for use on post mills from
the horizontal-axis water wheel developed by Vitruvius.

As early as 1390, the Dutch
set out to refine the tower mill design, which had appeared somewhat
earlier along the Mediterranean Sea (Figure 3, above left). The Dutch essentially affixed
the standard post mill to the top of a multi-story tower, with separate floors devoted to
grinding grain, removing chaff, storing grain, and (on the bottom) living quarters
for the windsmith and his family. Both the post mill and the later tower mill design had
to be oriented into the wind manually, by pushing a large lever at the back of the mill.
Optimizing windmill energy and power output and protecting the mill from damage by furling
the rotor sails during storms were among the windsmith's primary jobs.

Figure 4. An operating Dutch
windmill (1994) that features leading edge airfoil sections (at top right). The mechanism
used to turn the rotor into the wind and the windows of the first-floor living quarters
are easily seen.

A
primary improvement of the European mills was their designer's use of sails that generated
aerodynamic lift (see Figure 4 at the left). This feature provided improved rotor efficiency
compared with the Persian mills by allowing an increase in rotor speed, which also allowed
for superior grinding and pumping action.

The
process of perfecting the windmill sail, making incremental improvements in efficiency,
took 500 years. By the time the process was completed, windmill sails had all the major
features recognized by modern designers as being crucial to the performance of modern wind
turbine blades, including 1) camber along the leading edge, 2) placement of the blade spar
at the quarter chord position (25% of the way back from the leading edge toward the
trailing edge), 3) center of gravity at the same 1/4 chord position, and 4) nonlinear
twist of the blade from root to tip (Drees, 1977). Some models also featured aerodynamic
brakes, spoilers, and flaps. The machine shown in Figure 4 (which was operating with two
of its buddies pumping water about one meter up from one irrigation pond to another in the
Netherlands in 1994) features leading edge airfoil sections.

These mills were the
"electrical motor" of pre-industrial Europe. Applications were diverse, ranging
from the common waterwell, irrigation, or drainage pumping using a scoop wheel (single or
tandem), grain-grinding (again, using single or multiple stones), saw-milling of timber,
and the processing of other commodities such as spices, cocoa, paints and dyes, and
tobacco.

While continuing well into the 19th
century, the use of large tower mills declined with the increased use of steam engines.
The next spurt of wind power development occurred many thousands of miles to the west.

Figure 5. A steel-bladed
water pumping windmill in the American Midwest (late 1800's)

Role of Smaller
Systems

For hundreds of
years, the most important application of windmills at the subsistence level has been
mechanical water pumping using relatively small systems with rotor diameters of one to
several meters. These systems were perfected in the United States during the19th century,
beginning with the Halladay windmill in 1854, and continuing to the Aermotor and Dempster
designs, which are still in use today.

The first mills had four paddle-like wooden
blades. They were followed by mills with thin wooden slats nailed to wooden rims. Most of
these mills had tails to orient them into the wind, but some were weather-vaning mills
that operated downwind of the tower. Speed control of some models was provided by hinging
sections of blades, so that they would fold back like an umbrella in high winds, an action
which reduced the rotor capture area to reduce thrust. The most important refinement of
the American fan-type windmill was the development of steel blades in 1870 (Figure 5).
Steel blades could be made lighter and worked into more efficient shapes. They worked so
well, in fact, that their high speed required a reduction (slow-down) gear to turn the standard
reciprocal pumps at the required speed.

Between 1850 and 1970, over
six million mostly small (1 horsepower or less) mechanical output wind machines were
installed in the U.S. alone. The primary use was water-pumping and the main applications
were stock watering and farm home water needs. Very large windmills, with rotors up to 18
meters in diameter, were used to pump water for the steam railroad trains that provided
the primary source of commercial transportation in areas where there were no navigable
rivers.

In the late 19th century, the successful
"American" multi-blade windmill design was used in the first large windmill to
generate electricity.